As a chemical processing method which takes the place of mechanical processing and is capable of processing a surface of a substrate without producing a lattice defect, a so-called photoelectrochemical etching method is known, which performs etching of a surface of a substrate in an acidic or basic processing solution by irradiating the surface of the substrate with ultraviolet rays or by applying a potential bias to the substrate. The photoelectrochemical etching method, by the assistance of a light energy and an electrical energy, enables processing of a surface of a substrate only through a chemical action with little damage to the surface of the substrate. The photoelectrochemical etching method, however, is not generally suited for processing and flattening of a surface of a substrate because this method lacks a flattening reference and, in addition, involves defect selectivity, and the like.
Chemical mechanical polishing (CMP) is also widely known which uses a polishing liquid containing an abrasive, such as SiO2 or Cr2O3, and performs processing of a surface of a substrate by denaturing the surface of the substrate and mechanically removing the denatured layer. Because CMP involves a mechanical action, such a denatured layer cannot be fully removed by CMP. Further, it is generally difficult for CMP to process and flatten a surface of a Ga element-containing compound semiconductor substrate at a sufficient processing rate.
The applicant has proposed a catalyst-referred chemical processing method which comprises disposing a substrate in an oxidizing processing solution, disposing an acidic or basic solid catalyst in contact with or in close proximity to a surface (surface to be processed) of the substrate, and dissolving surface atoms of the surface to be processed, in contact with or in close proximity to the solid catalyst, in the oxidizing processing solution, thereby processing the surface to be processed (see Japanese Patent Laid-Open Publication No. 2008-121099). In this catalyst-referred chemical processing method, oxidation of the surface to be processed can be promoted and the processing rate can be increased by irradiating the surface of the substrate (object to be processed), disposed in the processing solution, with light, preferably ultraviolet light, or by applying a voltage between the substrate and the solid catalyst. This catalyst-referred chemical processing method enables processing of a surface of a substrate only through a chemical action with little damage to the surface of the substrate. It is, however, generally difficult for this method to process and flatten a surface of a Ga element-containing compound semiconductor substrate at a sufficient processing rate.